Tax reform is one of the few issues where almost every GOP candidate has issued a robust platform on their campaign website. Many of the 2016 candidates believe that Americans pay too much in taxes and the code needs to be simplified.

But is it a question of pulling back existing rates or moving to another system all together?

Marco Rubio, Jeb Bush, Chris Christie, Donald Trump and others have previously proposed cutting current tax rates to 8 percent to 35 percent, depending on the bracket, with various reforms to marriage and child credits and capital gains, corporate and estate taxes. Candidates like Ted Cruz, Ben Carson and Rand Paul said they support replacing the current tax system with a simpler form of flat tax.

Mike Huckabee called these plans "putting lipstick on a pig" and told CNBC he would promote replacing income tax (and the IRS) with a higher sales tax.

— CNBC's John Harwood and Anita Balakrishnan contributed to this report.